1. Field of the Invention
This invention relates to a leg-holding device for an offshore platform, especially for an oil drilling rig installed offshore.
2. Description of the Prior Art
An offshore platform is provided with several legs that ascend and descend vertically and is installed in a specified place on the sea by lowering the legs onto the seabed after it is towed to the specified place. To ensure a stable installation condition despite the effect of strong waves and under severe oceanic conditions, it is necessary that the legs lowered onto the seabed positively support not only the whole weight of the platform but also the environmental load. Usually, a leg is of a truss structure composed of three or four columns and each column has on its side racks which extend longitudinally. A lifting device having several pinions that mesh with these racks is mounted on the deck of the offshore platform. Each pinion is turned by means of a motor to lower the leg, and when all the legs have reached the seabed the deck is lifted along the legs. When the legs are positively secured on the seabed by the weight of the deck, the deck is immovably held against the legs by fixing the meshing of the racks with the pinions, which is accomplished by braking the motors. In a prior art apparatus, the lifting device also has the function of leg holding, a hydraulic cylinder is operatively connected to each motor, and the working chambers of the hydraulic cylinders are interconnected to make uniform the torques acting on respective pinions during leg holding, as disclosed in Japanese Patent Laid-Open No. 86599/84.
Under very severe environmental conditions, however, the leg-holding capability may be insufficient only through the leg-holding action of a lifting device. In another prior art apparatus, therefore, a leg-holding mechanism having stoppers that engage with racks aside from pinions is installed on the deck of an offshore platform, and this leg-holding mechanism performs the leg-holding function separately. Furthermore, in another example of this prior art apparatus, the leg-holding capability of the lifting device is also utilized and the leg-holding action is shared by the lifting device and the leg-holding mechanism, as disclosed in Japanese Patent Laid-Open No. 188514/85.
In the apparatus disclosed by Japanese Patent Laid-Open No. 188514/85, a large-capacity leg-holding mechanism is used for the sake of safety because no consideration is given to the proportions of load to be shared by the lifting device and leg-holding mechanism.